The present invention relates generally to a dot-line printer, and more particularly to a printing head for use in such a printer. In the dot-line printer, a hammer bank is reciprocally moved along a printing line to thereby carry out the printing of characters, symbols, etc. The hammer bank includes a plurality of dot-printing hammers separately provided at equal pitches along the printing line.
FIG. 1 shows one example of an arrangement of the printing hammers, in which each of the hammers is made up of a leaf spring 2 and a printing pin 1 attached to the upper end of the leaf spring. The lower end of each of the printing hammers is fixedly secured to a fixing member 3 by means of, for example, a screw (not shown). The printing hammers thus assembled are accommodated in the hammer bank and are reciprocated in forward and backward directions. The single forward or backward movement of the hammer bank is called "shuttle movement".
FIG. 2 shows a cross-sectional side view showing a printing hammer driving device. The leaf spring 2 is attracted to a pole of a yoke 5 by a permanent magnet 4, and when a release coil 6 is energized, the leaf spring 2 is released and the printing pin 1 strikes a paper through an ink ribbon (not shown). Since the printing pins 1 are juxtaposed along the printing line, printing of one dot-line is achieved by one shuttle movement of the hammer bank. If one character is formed with 24.times.24 dot matrix, it is necessary for the hammer bank to perform twenty-four shuttle movements. Therefore, there has been a limitation in increasing the printing speed.
In order to increase the printing speed, it has been proposed in the copending U.S. patent application Ser. No. 124,121 filed Nov. 23, 1987 to displace the position of the adjacent printing pins in the paper feeding direction by, for example, one dot-line. With N-number printing pins thus displaced, N dot-lines can simultaneously be printed. One example of such a hammer arrangement is illustrated in FIG. 3, with which 6 dot-lines are simultaneously printed with one shuttle movement of the hammer bank. According to such an arrangement of the hammers, printing of one line can be carried out with four (4) shuttle movements of the hammer bank.
While the arrangement of the printing hammers in FIG. 3 is advantageous in that the printing speed is increased, it is disadvantageous in that the leaf springs 2 have variations not only in spring constants but also in repeatability and operational characteristics of the striking forces. This is due to the fact that the distance from the fixing member 3 to the printing pin 1 is unequal in the respective print hammers. There exists five (5) dot-lines difference between the printing hammer in which the printing pin 1 is attached to the furthest position from the fixing member 3 and a print hammer in which the printing pin is attached to the nearest position from the fixing member 3. When the printing is carried out with the print hammers as in FIG. 3, the printing speed is restricted by the printing hammer of the worst repeatability. For this reason, a limitation still exists in increasing the printing speed.
In view of the disadvantages accompanying in such prior art print hammers, it has been proposed as illustrated in FIG. 4 to attach a plurality of print hammer modules to a module attachment member 8 through a holder 7. Each hammer module contains a predetermined number of printing hammers (four in FIG. 4). The printing hammers are mounted in inclined manner with respect to the attachment member 8. In the hammer module, the distance between the fixing member 3 and the printing pin 1 of each of the printing hammers is made substantially equal to one another. According to the arrangement shown in FIG. 4, while the operational characteristics of the respective printing hammers can substantially be made equal to one another, another disadvantages are introduced such that the structure is complicated and the cost of the dot-line printer becomes expensive due to the increase of the number of the components.
Further, it has been known in the art that for the purpose of preventing a magnetic interference and suppressing vibrations of a mechanical frames, every other printing hammers are mounted so as to be displaced by an amount corresponding to one-half of a dot printing pitch (hereinafter referred to as "a half-dot") in the direction perpendicular to the paper feeding direction. Referring to FIG. 6, the printing pins are to be mounted at equal pitches P, but every other printing pins are displaced by the amount of the half-dot in the left direction. The displaced printing pins are indicated with black circles and the normally placed printing pins are indicated with white circles. In the printing pins thus arranged, all the printing hammers are not simultaneously fired, so that the magnetic interference can be prevented and the impact occuring at the time of striking the printing pins against the paper is decreased, whereby the mechanical vibrations are suppressed.
In the printing hammer shown in FIG. 5, since it is necessary that every other printing hammers be displaced by the amount of the half dot, fabrication of the printing hammers in such a fashion is troublesome and thus the final product becomes costly.